This invention relates to the coating of steel sheet with a corrosion resistant nonferrous alloy. It relates particularly to a electrodeposited coating of a steel sheet with a zinc-manganese alloy.
It is well-known that steel sheet can be protected from corrosion by a nonferrous metallic coating, such as aluminum, tin or zinc. It is also well-known that alloys of nonferrous metals, such as zinc-nickel alloy or layers of different nonferrous metals, such as zinc and chromium can be used to coat steel to improve its corrosion resistance and other properties, such as paintability. Alloy coatings have wide application in the automotive industry to protect automotive components from corrosion.
Zinc-manganese alloy coatings have been electroplated on steel sheet with generally good results. A process for electroplating steel sheet with a single layer of zinc-manganese alloy coating is described in a paper entitled "Electrodeposition of Zinc-Manganese on Steel Strip" by M. Sagiyama, et al., appearing in the November, 1987 issue of Plating and Surface Finishing. Society of Automotive Engineers Paper No. 860268 (1986) entitled "Zinc-Manganese Alloy Electroplated Steel for Automotive Body" by M. Sagiyama, et al., further describes the properties of a single layer zinc-manganese alloy coating on sheet steel for automotive applications.
These papers describe that single layer zinc-manganese coatings (30-50%) manganese have good corrosion resistance, both before and after painting.
One problem with many nonferrous alloy coatings is the tendency of such coatings to "powder" when the coated steel sheet is being formed or fabricated. Powdering is characteristic of a number of coatings in which portions of the coating crack and flake off the surface of the coated steel sheet during the stamping or forming of the coated sheet. Not only does this result in a partial loss of the protective coating and possibly tiny cracks in the coating, but also the "powder" tends to buildup in the dies used during the stamping or forming of the coated sheet. The accumulated powder in the dies can then cause imperfections in parts subsequently stamped or formed.